`LinuxPod`: add stdin/stdout/stderr attach to an already-running process (PTY re-attach)

[SCKelemen]

Problem

The Containerization framework provides LinuxPod.execInContainer(_:processID:configure:)
to spawn a new process inside a running container, but exposes no mechanism
to re-attach an interactive PTY (stdin + stdout + stderr) to the primary
process that was started with startContainer(). Once the initial stdout/stderr
streams are consumed at start time, there is no way to reconnect.

This means the Kubernetes CRI Attach RPC — which connects a client's terminal
to the container's existing entrypoint process — cannot be implemented on
the pod-shared path.

Use case

kubectl attach -it <pod> expects the CRI runtime to open a bidirectional
byte stream to the container's primary process. On the pod-shared path
(podvmd), the CRI runtime dispatches a JSON-RPC pod.attach call to the
daemon. The daemon must then re-attach to the process that was started at
container boot.

Without this primitive, the only option is to exec a new shell, which is
semantically different (new PID, new process group, does not share the
entrypoint's stdin history or signal handling).

Reproducer

In our podvmd daemon (tools/podvmd/Sources/podvmd/PodRegistry.swift,
line ~449):

// MARK: - pod.attach
//
// Apple Containerization framework (as of 0.12.x) does not expose a
// re-attach primitive for the primary container process.

func attach(_ params: PodAttachParams) async throws -> PodAttachResult {
    _ = try podEntry(params.handle)
    throw PodVMError(
        .methodNotFound,
        "pod.attach not supported by Apple Containerization framework on this host"
    )
}

The Go CRI layer (darwin-cri) calls this method and receives JSON-RPC error
code -32601 (method not found), which it maps to gRPC codes.Unimplemented:

Attach: pod-shared path: pod.attach returned method_not_found
        (-32601: pod.attach not supported by Apple Containerization
        framework on this host)

A kubectl attach -it against a pod-shared pod produces:

error: unable to upgrade connection: container attach not supported

Proposed API shape

Option A — method on LinuxPod:

/// Attach stdin/stdout/stderr to an already-running container process.
/// Returns when the client disconnects or the process exits.
func attach(
    containerID: String,
    processID: String?,       // nil = primary process
    stdin: (any ReaderStream)?,
    stdout: (any Writer)?,
    stderr: (any Writer)?,
    tty: Bool
) async throws

Option B — method on LinuxProcess:

/// Re-open the stdio streams for an existing process.
func openTTY() async throws -> (stdin: FileHandle, stdout: FileHandle)

Either shape would unblock the CRI Attach path.

Workarounds we've tried

1. Exec a new /bin/sh — not semantically equivalent; creates a new
   process rather than attaching to the existing one.
2. Capture stdout/stderr at start time into files, then tail — provides
   read-only output but no stdin, and misses data written before the tail
   starts.
3. Hold the initial Writer references open — works for log streaming
   but does not provide a way to inject stdin after the fact, and the
   Writer protocol is unidirectional.